During these 20 years, various low molecular liquid crystalline materials have been synthesized and used as optical display elements for watches, computers, televisions and the like. Regarding to high molecular liquid crystalline materials, attention has been directed to studies on thermotropic liquid crystalline materials and especially on such resins as having improved strength and thermal resistance under normal conditions and being easily transformed to molten phase due to liquid crystal properties thereof, as in all aromatic polyester resins disclosed in Japanese Patent Application Kokai Nos. 54-50594 and 55-144024. Other high polymers having thermotropic liquid crystal properties have been stated in Eur. Polym. J. 16, 303, 1980 by D. Van Luyen et al, Polymer 24, 1299, 1983 and Kobunshi Kako 34, [1], 39, 1985 by W. R. Kringbaum and J. Watanabe. In these high polymers, liquid crystal properties (Optical anisotropic properties) are generated, as in low molecular liquid crystalline materials, by the repeating unit structure in the main chain comprising a stiff mesogenic group and a soft spacer group. Since the abovementioned main chain type, high molecular liquid crystalline materials have, in general, a large orientation time of molecular motion, they are not always suitable for a high response display application in contrast to low molecular liquid crystalline materials. However, research activities have been continued to apply them to thermal recording medium, wave-selective transmission or reflecting film and the like to make the most of their film-forming and filament forming properties.
However, heretofore proposed liquid crystal polyesters are all of thermoplastic nature. Their end groups are, in general, blocked ester bonds as acetyl, phenylester and the like. This is because if there exist end functional groups, the polyesters are liable to be pyrolized at the melting points as high as 250.degree..about.400.degree. C. Their average molecular weights are also very high, e.g. tens of thousands to several hundred thousands, so as to be suitable for fluxing. Thus, the proposed polyesters do not contain any or any substantial quantities of end functional groups (if any, it is only in the level enough to give a resinous acid value of less than 10), and therefore they can not be used as a thermosetting resin for coating composition.
Under the circumstances, such polyester resin as being useful as a thermosetting resin and capable of forming an anisotropic molten phase at a lower melting point has been longed for, in paint industries.
In a coating composition area, a thermosetting polyester resin has become the object of public attention as the main constituting component of powder coating resinous composition.
Coating powders are usually prepared by mixing resinous component, pigment and other additives, kneading the mixture at an elevated temperature, cooling the mixture to a solid mass and pulverizing the same. Therefore, if the employed resinous component is of thermosetting nature, it must be stable at the melt-kneading stage, but fully cured by heating at the curing stage.
In considering powder coating resinous composition comprising a polyester resin and a melamine resin or blocked polyisocyanate compound, most of the polyester resins proposed for solvent type coating composition's are possessed of lower softening points and therefore, when powder coating is prepared with said resins, blocking of the powder is liable to occur during storage of said powder. When a polyester resin with a higher softening point is selected, such resin is easily gelated at the melt-kneading stage, and therefore, great difficulties are encountered in the preparation of coating powder. Even succeeded in obtaining the powder, there are problems of poor leveling and gloss of the formed coating.
In U.S. Pat. No. 4,352,924 and EP 0,070,118, are disclosed thermosetting powder coating compositions with crystalline polyesters prepared from terephthalic acid, isophthalic acid and 1,4-cyclohexane dicarboxylic acid as dicarboxylic acid components and 1,4-butanediol and 1,6-hexanediol as diol components. However, it is not yet up to the mark in the sense of striving for compatibility of blocking resistance of the powder and leveling of the formed coating.
Furthermore, powder coating comprising a carboxyl containing polyester resin and a polyepoxy compound has also been known in the art, as, for example, Japanese Patent Publication No. 55-15506 and Japanese Patent Publication No. 58-29342. Such powder coating is excellent in mechanical strength, adhesion and anticorrosion properties and therefore has been widely used in various technical fields. Finally, JP 57-198726 discloses polyesters derived from 4,4-diphenyl dicarboxylic acid and alkyleneglycol, but such polyesters are incapable of giving an anisotropic molten phase and, further, these polyesters have a transition point of 250.degree. C. or more.
However, the disclosed powder coatings are all based on polyester resins with considerably higher acid values, e.g. 40.about.70, and therefore, there are problems of inferior storage stability of the powder and poor leveling of the formed coating.
It is, therefore, an object of the invention to provide a novel thermosetting polyester resin which can be used as a thermosetting resin in the field of coating composition and especially coating powder and can be transformed to an anisotropic molten phase at a considerably lower melting temperature. It is an additional object of the invention to provide a thermosetting resinous composition comprising the abovementioned polyester resin and a hardener, which can be used as the base material for coating powder. A further object is to provide coating powder which is free from undesired blocking during storage of the powder, easily prepared by a conventional method of melt kneading and capable of resulting in a coating with excellent leveling and gloss properties.